This appeared today. This may help you for changes in future.
Date: 2012-10-06 00:48:04
ID: 1
Error occured in: Global
Count: 1
Message: ...ace\AddOns\ncImprovedMerchant\ncImprovedMerchant.lua line 12:
attempt to perform arithmetic on a nil value
Debug:
...ace\AddOns\ncImprovedMerchant\ncImprovedMerchant.lua:12:...

The error seems to be with this values:
nt:SetAllPoints(bu)
--applying color
nt:SetVertexColor(cfg.color.normal.r,cfg.color.normal.g,cfg.color.normal.b,1)
Lines 222 and 225.
I just put "--" in codes and its 100% now.